Recently, inkjet printing systems have come to be widely used in various image forming apparatuses, such as printers, facsimiles, photocopiers, and multifunctional machines having image forming capabilities. In particular, an inkjet printer includes a print head to form an ink image by ejecting droplets of ink onto a recording medium or recording sheet conveyed by a media transport mechanism throughout the printing process.
In some inkjet printers, the media transport mechanism is implemented as an endless transport belt supported by and tensioned around multiple rollers defining a travel path of the recording medium. As the supporting rollers rotate, the transport belt moves along the travel path while conveying thereon a recording sheet through a print zone, i.e., beneath the print head, where ink droplets ejected from the print head land on the conveyed recording sheet.
In order for ink droplets to land at desired locations of the recording sheet, it is important to maintain a consistent gap between the conveyed sheet and the print head. Achieving good flatness of a recording sheet during printing is therefore highly required in such inkjet printers, while not so in a laser printer which transfers an image onto a recording sheet by direct contact with a photoconductive surface. For this reason, media transport used in an inkjet printer is typically equipped with a source of suction or electrostatic force to attract the recording sheet onto the moving belt and maintain it in a flat, stable condition.
One problem that affects belt-based media transport systems is local deformation of the transport belt occurring under certain usage conditions. That is, when a transport belt is held stationary under tension for extended time periods, a portion of the belt remaining in contact with the supporting roller conforms to the curve of the roller surface, arching outwardly from a normal position and maintaining that curve even after separating from the roller. Such belt deformation is known to adversely affect the imaging performance of the inkjet printer using the belt transport.
To take a specific example, as the transport belt rotates and a local deformation is advanced to the print zone upon such rotation of the transport belt, the outward arch of the deformation might accidentally contact an ink ejecting face of the print head. Moreover, a recording sheet conveyed on such an outward deformation is occasionally displaced from the belt surface to undesirably interfere with the print head in the print zone. Such interference between the sheet and the print head leads to various printing defects, such as sheet misalignment or paper jam and improper placement of ink droplets on the sheet, which degrade imaging quality of the image forming apparatus.
To cope with the belt deformation of the media transport, various techniques have been proposed.
For example, one conventional method proposes a chargeable transport belt with electrode arrays implanted on a back side thereof. The backing electrode arrays provide increased electrostatic force to firmly attract a recording sheet onto the transport belt. Another conventional method provides a transport belt with a source of variable attraction force, which exerts a relatively large force when a recording sheet is conveyed in an abnormal condition.
These conventional techniques are designed to stabilize media transport with a transport belt, and although capable of avoiding defects caused by a damaged belt, do not provide a fundamental solution to the problem, namely, one that can correct belt deformation. It is therefore advantageous to have a belt transport system that can correct local deformation of a transport belt, achieving reliable conveyance of material without interfering with neighboring components. An inkjet printer having such a belt transport will provide enhanced imaging quality with stable ink ejecting performance.